Natural fiber-filled polyolefin composites

ABSTRACT

A process for preparing a composite material is disclosed wherein the process comprises:  
     A) sizing a natural fiber with a reactive organosilane;  
     B) mixing the sized natural fiber with a polyolefin resin; and  
     C) adding a functionalized polyolefin coupling agent to the mixture of the sized natural fiber and the polyolefin resin to provide said composite material.

[0001] We claim the benefit under Title 35, United States Code, § 120 ofU.S. Provisional Application No. 60/416,962, filed Oct. 9, 2002,entitled NATURAL FIBER-FILLED POLYOLEFIN COMPOSITES.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to polyolefin composites comprisingnatural fibers. More particularly, the present invention relates tonatural fiber-filled polyolefin composites having improved mechanicalproperties resulting from the inclusion of a reactive organosilane and afunctionalized polyolefin coupling agent during their manufacture.

[0004] 2. Description of Related Art

[0005] It is known in the art to prepare composites comprisingthermoplastic resinous matrix materials having dispersed therein organicreinforcing fillers, such as cellulosic or lignocellulosic fibers. It isalso known in the art to improve the mechanical properties of suchcomposites by treating such fibers with organosilanes or, alternatively,other coupling agents prior to their introduction into the thermoplasticresinous matrix material.

[0006] U.S. Pat. No. 4,717,742 discloses resin composites reinforcedwith silanes grafted onto organic fillers that are said to have improveddurability, even at sub-zero degrees or at high temperatures, improvedphysical properties and can be prepared by a process, in which theorganic filler is grafted with a silane coupling agent in maleatedpolymer matrix.

[0007] U.S. Pat. No. 4,820,749 discloses a composite material based on apolymeric or copolymeric substance which may be a thermoplastic orthermosetting material or rubber, and all organic material which iscellulosic or starch. The cellulosic material is grafted with asilylating agent. Processes for preparing this composite are alsodisclosed.

[0008] U.S. Pat. No. 6,265,037 discloses an improved compositestructural member comprising a complex profile structural member, madeof a composite comprising a polypropylene polymer and a wood fiber. Thematerial is said to be useful in conventional construction applications.

[0009] U.S. Pat. No. 6,300,415 discloses a polypropylene composition forthe production of various molded articles which are said to be excellentin moldability, mold shrinkage factor on molding, rigidity, flexibility,impact resistance, in particular low-temperature impact resistance,transparency, gloss, stress-whitening resistance, and the balancethereof; various molded articles having the above properties; apropylene composition which is suitable for a base resin for thepolypropylene composition; and a process for the production thereof. Thepropylene composition comprises a propylene homopolymer and apropylene-ethylene copolymer

[0010] Kokta, B. V. et al., Polym.-Plast. Technol. Eng., 28(3):247-259(1989) studied the mechanical properties of polypropylene with woodflour. The wood flour was pretreated with polymethylenepolyphenylisocyanate and silane coupling agents before adding it to thepolymer.

[0011] Raj, R. G. et al., Polym.-Plast. Technol. Eng., 29(4):339-353(1990) filled high density polyethylene with three different cellulosicfibers that had been pretreated with a silane couplingagent/polyisocyanate to improve the adhesion between the fibers and thepolymer matrix.

[0012] Matuana, L. M. et al. ANTEC 3:3313-3318 (1998) studied the effectof the surface acid-base properties of plasticized PVC and cellulosicfibers on the mechanical properties of the plastic/cellulosic composite.They modified the surface of the fibers withγ-aminopropyltriethoxysilane, dichlorodiethylsilane, phthalic anhydride,and maleated polypropylene.

[0013] The disclosures of the foregoing are incorporated herein byreference in their entirety.

SUMMARY OF THE INVENTION

[0014] According to the present invention, the mechanical strengthproperties of natural fiber-filled polyolefin composites are improved bytreating (sizing) the fiber with a reactive organosilane and, then,adding a functionalized polyolefin coupling agent to the polyolefinresin during the mixing of the fiber and polyolefin resin.

[0015] More particularly, the present invention is directed to a processfor preparing a composite material comprising:

[0016] A) sizing a natural fiber with a reactive organosilane;

[0017] B) mixing the sized natural fiber with a polyolefin resin; and

[0018] C) adding a functionalized polyolefin coupling agent to themixture of the sized natural fiber and the polyolefin resin to providesaid composite material.

[0019] In another aspect, the present invention is directed to acomposite material prepared by a process comprising:

[0020] A) sizing a natural fiber with a reactive organosilane;

[0021] B) mixing the sized natural fiber with a polyolefin resin; and

[0022] C) adding a functionalized polyolefin coupling agent to themixture of the sized natural fiber and the polyolefin resin to providesaid composite material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] It is often desirable to increase the strength properties ofnatural fiber-filled polyolefin composites, e.g., wood-polyolefincomposites, for construction and automotive applications. It is known touse functionalized polyolefin coupling agents to increase such strengthproperties.

[0024] It has now been found that by using a natural fiber sized with areactive organosilane in combination with a functionalized polyolefin inthe resin component of the natural fiber-plastic composite synergisticincreases in strength properties can be obtained, as compared toformulations using a sized fiber alone or formulations using a couplingagent alone. These improvements allow the use of naturalfiber-polyolefin composites in applications, such as deck supports,railing systems, and automotive parts where structural properties areimportant. It has also been found that the combination of the reactiveorganosilane sized fiber and the functionalized coupling agent alsoprovides composites having improved long-term durability by reducingwater absorption and increasing creep resistance.

[0025] As employed herein, the term “natural fiber” means a fiberobtained, directly or indirectly, from a source in nature. Includedwithin the term, but not limited thereto, are wood flour, wood fiber,and agricultural fibers such as wheat straw, flax, hemp, kenaf, nutshells, and rice hulls. Preferably, the natural fiber is selected fromthe group consisting of starch or cellulosic material such as cottonfibers, wood pulps, stem or vegetable fibers, wood flours, starch, wastepapers, cartons, or cellulosic cloth. More preferably, the natural fiberis wood flour, wood fiber, hemp, flax, or kenaf. Wood fiber, in terms ofabundance and suitability, can be derived from either soft woods orevergreens or from hard woods commonly known as broadleaf deciduoustrees. While soft wood and hard wood are preferably the primary sourcesof fiber for the invention, additional fiber make-up can be derived froma number of secondary or fiber reclaim sources, including hard woods,bamboo, rice, sugar cane, and recycled fibers from newspapers, boxes,computer printouts, and the like. However, the primary source for woodfiber used in the process of this invention comprises the wood fiberby-product of sawing or milling softwoods and hardwoods commonly knownas sawdust or milling tailings. Fiber levels in the range of from about20 to about 85 weight % can be used. Fiber levels in the range of fromabout 30 to about 80 weight % are preferred. Fiber levels in the rangeof from about 40 to about 70 weight % are most preferred.

[0026] The polyolefins employed in the practice of the present inventionare typically polymerized from ethylene, propylene, and/or other alphaolefins. Where ethylene is used, it can be, for example, HDPE, LDPE, orLLDPE. Polypropylene homopolymer, as well as copolymers and terpolymerscontaining ethylene, propylene, and/or other alpha olefins, and/ornon-conjugated dienes can also be advantageously employed, as can blendsof these polymers.

[0027] Thus, the polyolefin materials of the invention can, if desired,comprise either a polypropylene copolymer wherein the polymer comprisesa major proportion of propylene combined with a minor proportion(typically less than 50 wt %, more commonly between about 0.1 and 10 wt%) of a second monomer that can comprise ethylene or a C₄-C₁₆ monomermaterial. Such copolymers often have improved processability,flexibility, and compatibility.

[0028] Preferred ethylene copolymers can comprise a major proportion ofethylene and a minor proportion (typically less than 50 wt %, preferablyabout 0.1 to about 10 wt %) of a C₃-C₁₈ monomer.

[0029] Polypropylene homopolymer and HDPE, i.e., high densitypolyethylene, are most preferred for use in the practice of the presentinvention.

[0030] The functionalized polyolefin, which is preferably a modifiedpolyethylene or polypropylene, is one that contains reactive groups thatcan react with a functional group on a reactive organosilane. Suchpolymers are modified by a reactive group including at least one polarmonomer selected from the group consisting of ethylenically unsaturatedcarboxylic acids or ethylenically unsaturated carboxylic acidanhydrides. Mixtures of the acids and anhydrides, as well as theirderivatives, can also be used. Examples of the acids include maleicacid, fumaric acid, itaconic acid, crotonic acid, acrylic acid,methacrylic acid, maleic anhydride, itaconic anhydride, and substitutedmaleic anhydrides. Maleic anhydride is preferred. Derivatives that mayalso be used include salts, amides, imides, and esters. Examples ofthese include, glycidyl methacrylate, mono- and disodium maleate, andacrylamide. Virtually any olefinically reactive residue that can providea reactive functional group on a modified polyolefin polymer can beuseful in the invention. Preferably, such compatibilizers comprise apolyolefin, such as a polyethylene or polypropylene, having a weightaverage molecular weight (by GPC) that ranges from about 2000 to about400,000. Each polymer of the compatibilizing agent can be modified fromabout 0.1 to about 800 residues per mole of the polymer. Preferredcompatibilizers comprise either a modified polypropylene or a modifiedpolyethylene modified with maleic anhydride residues. The most preferredcompatibilizers are maleic anhydride modified polypropylenes and maleicanhydride modified high density polyethylenes. The preferred materialshave a weight average molecular weight (by GPC) that ranges from about20,000 to about 300,000 and have from about 0.6 to 310 moles of maleicanhydride per mole of polymer. A good example of a functionalizedpolyolefin is Polybond 3200, a maleic anhydride functionalizedpolypropylene, available from Crompton Plastics Additives Division.Functionalized polyolefin levels of 0.5 to 10% based on the totalformulation weight can be used with levels of 1-5% being preferred.

[0031] Any organosilane that is capable of reacting with both thefunctional groups on the surface of the natural fiber and the reactivesite on the functionalized polyolefin can be employed in the practice ofthe present invention. For example, organosilanes, such as aminosilanes,epoxysilanes, alkoxysilanes, methacrylic silanes, mercaptosilanes,chlorosilanes, and their oligomers, as well as mixtures and blendsthereof, can be used. Preferred organosilanes includeγ-aminopropyltriethoxysilane, γ-methacryloxypropyl trimethoxysilane, andpropyl triethoxy silane. However, this invention is not limited to suchsilanes. They may advantageously be replaced by other kinds or otherweight ratios of silylating agents, for example, vinyltriethoxysilane,vinyltri(2-methoxy-ethoxy)silane,β-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane,γ-glycidoxypropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane,γ-aminopropyltriethoxysilane,n-β-(aminoethyl)-γ-aminopropyltrimethoxysilane,n-β-(aminoethoxyl)-γ-aminopropyltrimethoxysilane andγ-chloropropyltrimethoxysilane or any other silylating agent having thegeneral formula

[0032] or an oligomer thereof, wherein R₁, R₂ and R₃ are the same ordifferent and are selected from the group consisting of alkoxy, alkyl,alkenyl, cycloalkyl, aryl, aralkyl, and organo-functional moieties.

[0033] The most preferred organosilane for use in the practice of thepresent invention is γ-aminopropyltriethoxysilane, e.g., OSi A-1100.Other silanes can, of course, be used depending on the resin and fibersselected. Carriers, both liquid and solid, can be used in addition tothe silanes to aid in the functionalization of the fibers andsynergistic improvements enjoyed when used with functionalizedpolyolefins. Treatment levels can range from about 0.05 to about 3.5% byweight of the fiber with the preferred range generally between about 0.1and about 2.0% by weight.

[0034] In a preferred embodiment, the natural fiber is first treatedwith 1.0-1.5% aminosilane. The treatment can be either with a directsilane addition to the fibers using an appropriate mixing device or withappropriate carrier or diluent technology, followed by appropriatemixing and drying of the fibers. The treated fiber is then blended witha powdered polypropylene resin and Polybond 3200 coupling agent. Theblended ingredients are fed into the main hopper of a 30 mm Coperiontwin screw extruder. A strand of the compounded product is cooled in awater bath and pelletized.

EXAMPLE 1

[0035] The above method was used to generate the data appearing incolumn D of Table I below after drying the pellets overnight at 100° C.and injection molding them to make the specimens for mechanical propertytesting. TABLE I Comparison of Methods for Improving Properties of WoodFlour-Filled Polypropylene D A B C Aminosilane + Description ControlMA-PP Aminosilane MA-PP 4020 Wood Flour 50 50 4020 Wood Flour + 50 50A-1100 Naugard B-25 0.125 0.125 0.125 0.125 PB 3200 2 2 Fortilene HB9200(4MF) 49.875 47.875 49.875 47.975 Specific Gravity 1.069 1.078 1.0681.083 Unaged Tensile Properties Peak Stress, MPa 28.4 34.7 30.1 44.4 %Change vs. Control 100 122 106 156 Flexural Properties Modulus, MPa 47774835 4835 5284 % Change vs. Control 100 101 101 111 Strength, MPa 45.457.5 47.5 70.2 % Change vs. Control 100 127 105 155 Impact ReversedNotch Izod, J/m 64.3 70.8 61.9 78.0 % Change vs. Control 100 110 96 121Unnotched Charpy, J/m 133 132 140 153 % Change vs. Control 100 99 105115

[0036] 4020 Wood Flour is a 40 mesh soft wood fiber commerciallyavailable for American Wood Fibers, Inc.

[0037] Naugard B-25 is a blend of phenolic and phosphite antioxidants.It is added for process stability and is commercially available fromCrompton Corporation, Middlebury, Conn.

[0038] Fortilene HB 9200 is a homopolymer polypropylene with an MFR of 4and 230° C. and 2.16 kg and a density of 0.900 gm/cc. It is commerciallyavailable from BP Amoco Chemical Company, Naperville, Ill.

[0039] In the above table, tensile properties were measured per ASTMprocedure D638, while flexural modulus and strength were obtained usingD790. Impact testing followed D256.

EXAMPLE 2

[0040] Example 1 was repeated except that 40 parts of hemp weresubstituted for the 50 parts of wood flour employed therein. The resultsare shown in Table II. TABLE II 40% Hemp-Filled Polypropylene Effect ofFiber Sizing and Coupling Agents Sample A B C D Hemp-Untreated 40 40Hemp treated with 40 40 A-1100 Naugard B-25 0.15 0.15 0.15 0.15 Polybond3200 2 2 Fortilene HB-9200 59.85 57.85 59.85 57.85 Mixed in Brabenderand Compression Molded Specific Gravity 1.070 1.068 1.064 1.070 UnagedFlexural Properties Modulus, MPa 4042 4249 3940 4374 *Change vs. Control100% 105%  97% 108% Strength, MPa 52.0 72.2 53.1 78.4 *Change vs.Control 100% 139% 102% 151%

[0041] Alternative processing approaches could be used in the practiceof the present invention that would be equally effective. In mostproduction facilities, the natural fiber is dried either prior toaddition to the extruder or in the extruder prior to mixing with theresin. In the former case, the reactive organosilane can be sprayed ontothe fiber as it enters the dryer. An example of equipment that could beused in the latter case is the Davis-Standard Woodtrudere®. In thisextrusion equipment, the reactive organosilane could be sprayed onto thenatural fiber during the drying process. This approach would have theadded advantage of reducing fiber dust, a major problem with thisoperation.

[0042] The reactive organosilane could also be adsorbed on the surfaceof a polymeric carrier and introduced with the resin and functionalizedpolyolefin during the compounding step. Suitable carriers includepolyolefin resins or functionalized polyolefin products, which areavailable as small, porous beads (so called reactor flakes or beads).

[0043] Additionally, the silane could also be added directly to theextrusion or compounding process. In one example of this approach, theliquid silane could be metered into a twin screw extruder at a liquidinjection port using any of a number of different pumping devices.Experience has taught that the injection point should be located in theupstream section of the extruder subsequent to the main feeder where theresin and natural fiber are added. Injection should also be made in aconveying section of the extruder screw prior to the distributive anddispersive mixing elements and that a vacuum port is required downstreamto vent off any volatiles generated by the reaction of the organosilanewith the natural fiber. The coupling agent may be added downstream ofthe organosilane or with the resin and natural fiber in the main feeder.

[0044] In view of the many changes and modifications that can be madewithout departing from principles underlying the invention, referenceshould be made to the appended claims for an understanding of the scopeof the protection to be afforded the invention.

What is claimed is:
 1. A process for preparing a composite materialcomprising: A) sizing a natural fiber with a reactive organosilane; B)mixing the sized natural fiber with a polyolefin resin; and C) adding afunctionalized polyolefin coupling agent to the mixture of the sizednatural fiber and the polyolefin resin to provide said compositematerial.
 2. The process of claim 1 wherein the natural fiber isselected from the group consisting of wood flour, wood fiber, andagricultural fiber.
 3. The process of claim 2 wherein the natural fiberis selected from the group consisting of wood flour, wood fiber, hemp,flax, and kenaf.
 4. The process of claim 1 wherein the natural fiber isemployed at a level in the range of from about 20 to about 85 weight %.5. The process of claim 1 wherein the polyolefin resin is apolypropylene copolymer comprising a major proportion of propylenecombined with a minor proportion of a second monomer selected from thegroup consisting of ethylene and C₄-C₁₆ monomer materials.
 6. Theprocess of claim 1 wherein the polyolefin resin is an ethylene copolymercomprising a major proportion of ethylene and a minor proportion of atleast one C₃-C₁₈ monomer.
 7. The process of claim 1 wherein thepolyolefin resin is polypropylene homopolymer.
 8. The process of claim 1wherein the polyolefin resin is high density polyethylene.
 9. Theprocess of claim 1 wherein the functionalized polyolefin coupling agentis a modified polyethylene or polypropylene comprising reactive groupsthat can react with a functional group on a reactive organosilane. 10.The process of claim 9 wherein the reactive groups comprise at least onepolar monomer selected from the group consisting of ethylenicallyunsaturated carboxylic acids, ethylenically unsaturated carboxylic acidanhydrides, and derivatives of the foregoing.
 11. The process of claim10 wherein the polar monomer is selected from the group consisting ofmaleic acid, fumaric acid, itaconic acid, crotonic acid, acrylic acid,methacrylic acid, maleic anhydride, itaconic anhydride, substitutedmaleic anhydrides, and derivatives of the foregoing.
 12. The process ofclaim 11 wherein the polar monomer is maleic anhydride.
 13. The processof claim 1 wherein the functionalized polyolefin coupling agentcomprises a maleic anhydride modified polypropylene having a weightaverage molecular weight in the range of from about 20,000 to about300,000 and comprises from about 0.6 to 310 moles of maleic anhydrideper mole of polymer.
 14. The process of claim 1 wherein thefunctionalized polyolefin coupling agent comprises a maleic anhydridemodified high density polyethylene having a weight average molecularweight in the range of from about 20,000 to about 300,000 and comprisesfrom about 0.6 to 310 moles of maleic anhydride per mole of polymer. 15.The process of claim 1 wherein the reactive organosilane is selectedfrom the group consisting of aminosilanes, epoxysilanes, alkoxysilanes,methacrylic silanes, mercaptosilanes, chlorosilanes, and oligomers,mixtures, and blends thereof.
 16. The process of claim 1 wherein thereactive organosilane is selected from the group consisting of

γ-aminopropyltriethoxysilane, γ-methacryloxypropyl trimethoxysilane,propyl triethoxy silane, vinyltriethoxysilane,vinyltri(2-methoxy-ethoxy)silane,β-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane,γ-glycidoxypropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane,γ-aminopropyltriethoxysilane,n-β-(aminoethyl)-β-aminopropyltrimethoxysilane,n-β-(aminoethoxyl)-γ-aminopropyltrimethoxysilane,γ-chloropropyltrimethoxysilane, and silylating agents having the generalformula or an oligomer thereof, wherein R₁, R₂, and R₃ are independentlyselected from the group consisting of alkoxy, alkyl, alkenyl,cycloalkyl, aryl, aralkyl, and organo-functional moieties.
 17. Theprocess of claim 1 wherein the reactive organosilane isγ-aminopropyltriethoxysilane.
 18. A composite material prepared by aprocess comprising: A) sizing a natural fiber with a reactiveorganosilane; B) mixing the sized natural fiber with a polyolefin resin;and C) adding a functionalized polyolefin coupling agent to the mixtureof the sized natural fiber and the polyolefin resin to provide saidcomposite material.
 19. The composite material of claim 18 wherein thenatural fiber is selected from the group consisting of wood flour, woodfiber, and agricultural fiber.
 20. The composite material of claim 19wherein the natural fiber is selected from the group consisting of woodflour, wood fiber, hemp, flax, and kenaf.
 21. The composite material ofclaim 18 wherein the natural fiber is employed at a level in the rangeof from about 20 to about 85 weight %.
 22. The composite material ofclaim 18 wherein the polyolefin resin is a polypropylene copolymercomprising a major proportion of propylene combined with a minorproportion of a second monomer selected from the group consisting ofethylene and C₄-C₁₆ monomer materials.
 23. The composite material ofclaim 18 wherein the polyolefin resin is an ethylene copolymercomprising a major proportion of ethylene and a minor proportion of atleast one C₃-C₁₈ monomer.
 24. The composite material of claim 18 whereinthe polyolefin resin is polypropylene homopolymer.
 25. The compositematerial of claim 18 wherein the polyolefin resin is high densitypolyethylene.
 26. The composite material of claim 18 wherein thefunctionalized polyolefin coupling agent is a modified polyethylene orpolypropylene comprising reactive groups that can react with afunctional group on a reactive organosilane.
 27. The composite materialof claim 26 wherein the reactive groups comprise at least one polarmonomer selected from the group consisting of ethylenically unsaturatedcarboxylic acids, ethylenically unsaturated carboxylic acid anhydrides,and derivatives of the foregoing.
 28. The composite material of claim 27wherein the polar monomer is selected from the group consisting ofmaleic acid, fumaric acid, itaconic acid, crotonic acid, acrylic acid,methacrylic acid, maleic anhydride, itaconic anhydride, substitutedmaleic anhydrides, and derivatives of the foregoing.
 29. The compositematerial of claim 28 wherein the polar monomer is maleic anhydride. 30.The composite material of claim 18 wherein the functionalized polyolefincoupling agent comprises a maleic anhydride modified polypropylenehaving a weight average molecular weight in the range of from about20,000 to about 300,000 and comprises from about 0.6 to 310 moles ofmaleic anhydride per mole of polymer.
 31. The composite material ofclaim 18 wherein the functionalized polyolefin coupling agent comprisesa maleic anhydride modified high density polyethylene having a weightaverage molecular weight in the range of from about 20,000 to about300,000 and comprises from about 0.6 to 310 moles of maleic anhydrideper mole of polymer.
 32. The composite material of claim 18 wherein thereactive organosilane is selected.
 33. The composite material of claim18 wherein the reactive organosilane is selected from the groupconsisting of aminosilanes, epoxysilanes, alkoxysilanes, methacrylicsilanes, mercaptosilanes, chlorosilanes, and oligomers, mixtures, andblends thereof.
 34. The composite material of claim 18 wherein thereactive organosilane is selected from the group consisting ofγ-aminopropyltriethoxysilane, γ-methacryloxypropyl trimethoxysilane,propyl triethoxy silane, vinyltriethoxysilane,vinyltri(2-methoxyethoxy)silane,β-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane,(-glycidoxypropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane,(-aminopropyltriethoxysilane,n-β-(aminoethyl)-γ-aminopropyltrimethoxysilane,n-β-(aminoethoxyl)-γ-aminopropyltrimethoxysilane,γ-chloropropyltrimethoxysilane, and silylating agents having the generalformula

or an oligomer thereof, wherein R₁, R₂, and R₃ are independentlyselected from the group consisting of alkoxy, alkyl, alkenyl,cycloalkyl, aryl, aralkyl, and organo-functional moieties.
 35. Thecomposite material of claim 18 wherein the reactive organosilane is(-aminopropyltriethoxysilane.